Watch your mindset. Your going for pushing the blood up your penis to force the cells to expand. I have felt pain twice and stopped immediately. Once from stretching and once from jelging.
If a 100% jelg strength will cause injury and 80% will cause pain then your max threshold should be 75%.
There is a point in jelging where if you squeeze much harder it is to much which at certain points is easy to do,especially when your reaching the glands.